1. Field of Invention
This invention relates to the field of blood treatment devices having a blood purification element divided by a semipermeable membrane into two chambers, the first chamber being part of a dialysis fluid circuit and the second chamber being part of an extracorporeal blood circuit.
2. Description of the Prior Art
Such devices are used as hemodialysis machines in artificial kidney therapy. Through hemodialysis treatment, it is possible to withdraw excess water from a dialysis patient by means of a pressure gradient on a semipermeable membrane. On the other hand, substances such as urea and creatinine, which would otherwise be eliminated by healthy kidneys, are withdrawn from the patient according to a concentration gradient on the membrane. In the case of other substances—such as electrolytes—which are always present in blood in a certain concentration, concentrations corresponding to those in the blood of persons with healthy kidneys are usually used in the dialysis fluid.
Dialysis patients usually consume considerable quantities of potassium in their food intake between treatments. Excessive intake of potassium, in particular an accumulation over a long period of time
The extracellular potassium concentration has an influence on the electrical resting membrane potential of cells. Therefore, this concentration influences the degree of electric stimulability of all stimulable cells (muscles, myocardium, nerves). The effect on cardiac electro-physiology in particular is important. Abnormal values can lead to life-threatening arrhythmias. Therefore, the body regulates the potassium concentration within narrow limits. Under the pathophysiological conditions of renal failure, dialysis therapy must be conducted in such a way as to prevent critical states.
Normal potassium concentrations are in the range of 3.5-4.5 mmol/L. Hyperkalemia is the condition when levels exceed 5.5 mmol/L; consequences include ventricular arrhythmia, ventricular fibrillations or even cardiac arrest. If the potassium level is below 2.5 mmol/L, this is called severe hypokalemia. Symptoms here include muscular weakness, atrial and ventricular arrhythmia, states of confusion and disorientation. The transitional ranges of mild hyperkalemia (4.5-5.5 mmol/L) and mild hypokalemia (2.5-3.5 mmol/L) are often largely symptom-free.
The potassium concentration is subject to much more complex kinetics during dialysis than the urea concentration, for example. Although urea is present in approximately the same concentrations intracellularly and extracellularly, the total potassium concentration of approximately 3500 mmol (50 mmol/kg) has a very irregular distribution: approx. 98% is intracellular and only 2% is extracellular. This imbalance between intracellular and extracellular concentrations is also influenced by changes in the concentration of other extracellular molecules, in particular H+ ions, bicarbonate, glucose and insulin. Adrenergic stimulation and the aldosterone level also influence the concentration ratio. In addition, a strong rebound effect is observed, i.e., after the end of dialysis, the blood potassium concentration rises again considerably because of the time lag in the transfer from the intracellular space to the extracellular space.
Removal of approximately 90% of the quantity of potassium ingested since the previous treatment by intermittent dialysis is not always simple. Only a relatively low concentration gradient can be utilized. Initial potassium concentrations in dialysis patients are usually between approx. 3.6 and 7 mmol/L or in some cases even higher. The potassium concentration in the dialysis fluid is between 0 and 4 mmol/L, average concentrations of approx. 2 mmol/L being used in most cases. The usable gradients between blood and dialysis fluid therefore typically amount to 2-4 mmol/L at the start of dialysis, but are rapidly reduced during the treatment. Therefore, the total possible elimination of potassium is limited. If a larger quantity of potassium is ingested, there is also the risk that the total elimination may be too low. Therefore, patients are at risk of hyperkalemia, and this may be especially pronounced, in particular immediately before a dialysis treatment.
Using dialysis fluid having a low potassium concentration or even no potassium at all is not without risk. The potassium concentration may drop to a critically low level during dialysis, i.e., severe hypokalemia may be triggered (even if the total potassium level in the body is still adequate or even too high).
If the rate of potassium elimination is too high, it may lead to arrhythmia (J. P. Knochel: “Clinical Expression of Potassium Disturbances,” in The Regulation of Potassium Balance, 1st edition, edited by D. W. Seldin, G. Giebisch, 1989, pp. 207-240; E. G. Lowrie and N. L. Lew: “Death Risk in Hemodialysis Patients: The Predictive Value of Commonly Measured Variables and an Evaluation of Death Rate Differences Between Facilities,” American Journal of Kidney Disease 15, pp. 458-482 (1990)). If there is an increase in the rate of elimination, the ratio of intracellular to extracellular potassium concentrations becomes greater, which then results in hyperpolarization of cells and thus a decline in stimulability. Therefore, there could be an increase in the probability of arrhythmia.
Administration of large quantities of bicarbonate to the patient, in particular in the early phase of dialysis, causes a shift in potassium from the extracellular space to the intracellular space, which also contributes to a reduction in the extracellular concentration and thus increases the risk of hypokalemia (in addition to the contribution made by the elimination of potassium via the dialysis machine).
There have been proposals for using model simulation with dialysis treatments to permit conclusions to be drawn regarding the removal of certain constituents of blood, in which case the calculations are supplemented by blood samples (S. Stiller, H. Mann and F. Raab, “Microprocessor-based universal dialysis calculator for individualization of artificial kidney dialysis,” MEDINFO '80, edited by Lindberg/Kaihara, North-Holland Publishing Company, 1980, pp. 534-538).
U.S. Pat. No. 4,244,787 describes a device in which the concentration of a substance in the blood inlet line can be determined by a sensor in the dialysis fluid outlet line. At the same time, it is possible to determine the total quantity of the substance removed.
U.S. Pat. No. 4,508,622 describes a hemodialysis machine in which the electrolyte balance during a dialysis treatment can be determined by a sensor in the dialysis fluid discharge line and a similar sensor in the dialysis fluid inlet line, and feedback to the dialysis treatment is made possible. A similar machine is disclosed in European Patent EP 0 330 892 A2.